The present invention relates generally to chemical processing methods and equipment and more particularly to a method and apparatus for producing gaseous sulfur trioxide.
Gaseous sulfur trioxide (SO3) has many uses. In one such use, gaseous sulfur trioxide is reacted with an organic reactant (e.g. alkyl benzene) to produce a sulfonate which is used to make detergents. Gaseous sulfur trioxide is also used to condition flue gas (e.g. from power generating boilers) to facilitate the removal of fly ash from the flue gas.
Typically, SO3 is produced by reacting sulfur and air in a sulfur burner to produce a first mixture consisting essentially of sulfur dioxide (SO2) and air. This first gaseous mixture is then flowed into a catalytic converter where the SO2 in the first mixture is converted to SO3 to produce a second mixture consisting essentially of SO3 and air which is withdrawn from the converter and directed to a location where the SO3 in the second mixture is reacted with an organic reactant to produce a sulfonate (in one example of a use) or where the SO3 is used to condition flue gas to facilitate the removal of fly ash (in another example).
There is a temperature range (e.g. 780-850xc2x0 F. (416-454xc2x0 C.)) which is favorable to initiate the catalytic conversion of SO2 to SO3. When the temperature of SO2 in the first mixture is either above or below this temperature range, it is difficult if not impossible to initiate the catalytic conversion of SO2 to SO3. Generally, the first mixture (SO2 and air) has a temperature above the favorable temperature range when the first mixture exits the sulfur burner. As a result, the first mixture is conventionally subjected to cooling between the sulfur burner and the converter. Cooling is typically accomplished by flowing the first mixture through either a radiant cooler or a heat exchanger, for example. A mixture of SO2 and air which has been thus cooled enters the converter at a temperature within the range favorable for initiating the conversion of SO2 to SO3.
The minimum temperature for initiating catalytic conversion of SO2 to SO3 (the threshold or ignition temperature) varies with the catalyzing agent employed in the conversion process and can be in the range 380 to 420xc2x0 C. (715-788xc2x0 F.), for example. Once the conversion reaction is initiated (ignition), it can be sustained at temperatures which may drop below the ignition temperature.
The conversion of SO2 to SO3 is an equilibrium reaction (SO2+xc2xdO2⇄SO3). In a typical commercial process, the oxygen required to convert SO2 to SO3 is provided by the air in the first mixture (SO2 and air). The percentage of SO2 which can be converted to SO3 varies with temperature and with the concentration (partial pressure) of the gaseous initial reactants (SO2 and O2). The lower the temperature in the temperature range at which the conversion reaction occurs, the greater the conversion of SO2 to SO3. For a given concentration of reactants and assuming the conversion reaction proceeds to equilibrium, there is a theoretical conversion percentage of SO2 to SO3 at each temperature within the range at which conversion can be sustained. The conversion temperature range has maximum and minimum temperatures. Maximum theoretical conversion occurs at the minimum temperature at which conversion can be sustained. Depending upon the concentration of the reactants, maximum theoretical conversion can be 99% or more, at a minimum sustaining temperature of 400xc2x0 C. (752xc2x0 F.), for example. In conventional commercial processes, the actual conversion percentage (yield) is usually an approximation of the theoretical conversion percentage, i.e., slightly below the theoretical conversion percentage; the closeness of the approximation is influenced by a variety of parameters such as gas distribution in the porous bed containing the catalyzing agent, gas velocity through that bed, and the activity of the catalyzing agent.
As noted above, there is a maximum temperature at which the conversion reaction can be sustained, and the maximum sustaining temperature decreases as the conversion percentage increases. For example, depending upon the concentration of the initial reactants, at a temperature of about 600xc2x0 C. (1112xc2x0 F.) the conversion reaction reaches equilibrium when the theoretical SO3 percentage is about 70%; a lower temperature, e.g., about 480xc2x0 C. (896xc2x0 F.) or below, may be required to obtain a theoretical conversion of 95%, and a temperature of about 400xc2x0 C. (752xc2x0 F.) may be required to obtain a theoretical conversion of 99%. An example of the concentrations of reactants, for achieving the results described in the preceding part of this paragraph, comprises about 10.5 vol. % SO2 and 10.4 vol. % O2. Generally, at a given temperature, the theoretical conversion percentage increases as the initial SO2 percentage decreases and the initial O2 percentage increases.
The conversion of SO2 to SO3 is an exothermic reaction which generates a substantial amount of heat in turn raising the temperature of the gases flowing through the converter to a temperature close to or above the temperature at which conversion can be sustained. In addition, as the conversion reaction proceeds, the percentage of SO3 in the gaseous stream increases, in turn requiring a decrease in the temperature of the gaseous stream in order for further conversion to occur. These two factors, i.e., increasing temperature and increasing SO3 percentage, require cooling of the gaseous stream in order to further increase the percentage of SO3 in the gaseous stream.
Therefore, in order to convert all or substantially all of the SO2 to SO3, it has been conventional to conduct commercial conversion processes in two or more conversion stages with the partially converted gaseous mixture from one stage being subjected to cooling between that stage and the next stage. Typically, cooling has been accomplished by flowing the partially converted gaseous mixture through either a radiant cooler or a heat exchanger located outside of the converter vessel. Alternatively, the partially converted mixture is diluted between stages with a cooling fluid, such as cool air, which, in addition to cooling the partially converted gaseous mixture, necessarily reduces the concentration of SO2 and SO3 in the partially converted gaseous mixture and increases its volume.
Cooling between stages reduces the temperature of the gaseous stream to a temperature at which catalytic conversion can be initiated and then sustained for awhile, keeping in mind that as conversion once again proceeds, the temperature of, and the percentage of SO3 in, the gaseous stream both increase, eventually again producing impediments to further conversion, as described above.
A converter employing two conversion stages together with a single cooling stage therebetween can, under appropriate circumstances, convert up to about 97% of the SO2 to SO3. A gaseous mixture in which up to about 97% of the SO2 has been converted to SO3 is acceptable for use in the conditioning of flue gas. However, when the SO3 is to be employed as a sulfonating agent, it is oftentimes desirable to employ a gaseous mixture in which 99% (or more) of the SO2 has been converted to SO3. In such a case, the converter employs three conversion stages (or more) with a cooling stage between the first and second conversion stages and another cooling stage between the second and third conversion stages, etc.
A gaseous mixture consisting essentially of air and SO3 is usually cooled after it exits the converter and before the SO3 therein is employed as a sulfonating agent. Typically, the gaseous mixture exiting the converter would not be cooled, to any substantial extent, when the SO3 is employed as a flue gas conditioning agent.
An example of a conventional process for producing SO3 for use as a sulfonating agent is described in UK published patent application GB 2 088 350 A. An example of a conventional process for producing SO3 employed as a conditioning agent for flue gas is described in U.S. Pat. No. 5,244,642. The subject matters described in both of these publications are incorporated herein by reference.
There are drawbacks to the above-described processes for producing gaseous SO3. These drawbacks arise from the need to subject the first gaseous mixture to cooling between the sulfur burner and converter; the need to subject the gaseous mixture undergoing conversion to cooling between the conversion stages; and the need to provide the converter with a plurality of conversion stages. These needs entail substantial expenditures for cooling equipment and the attendant piping, and they enlarge substantially the space occupied by the SO3-producing equipment package.
The present invention avoids the shortcomings of the prior art processes and apparatuses described above by utilizing a method and apparatus which eliminates the need for cooling equipment between the sulfur burner and the converter and between conversion stages of the converter.
In accordance with the present invention, the first mixture, consisting essentially of sulfur dioxide and air, is flowed from the sulfur burner directly to the catalytic converter without cooling the first mixture between the sulfur burner and catalytic converter. The catalytic converter comprises a vessel containing a plurality of spaced-apart channels each having upstream and downstream ends and each containing an agent for catalyzing the conversion of SO2to SO3. The converter also contains an upstream manifold, at the upstream ends of the channels, for receiving the gaseous mixture comprising SO2 and air and for directing portions of the first mixture into the upstream ends of the channels to form a plurality of streams containing the first mixture at the upstream channel ends.
The SO2 in the first mixture is converted to SO3, as the streams flow through the channels, to produce in the streams a second mixture consisting essentially of SO3 and air at the downstream ends of the channels. Each of the streams is cooled substantially continuously as it flows through its channel and as the SO2 therein undergoes conversion to SO3. Located within the converter, at the downstream ends of the channels, is a downstream manifold for receiving and combining the streams as they flow out of the channels.
Cooling of the streams as they flow through the channels is performed without introducing a cooling medium into any of the streams and without diverting any of the streams outside of the channels in the vessel. Each stream is cooled substantially immediately upon entering the upstream end of a channel and is subjected to cooling substantially continuously along substantially the entire length of the channel from its upstream end to its downstream end. The channels are defined by a multiplicity of spaced-apart tubular members which are cooled by contacting the exterior surface of each tubular member with a fluid cooling medium (e.g. cool air) along substantially the entire length of the tubular member, from its upstream end to its downstream end.
Each channel comprises an initial, upstream cooling portion and a single, continuous, uninterrupted conversion stage having upstream and downstream ends. The conversion stage terminates, in most embodiments, at the downstream channel end and contains all of the catalyzing agent to which the SO2 is subjected in the converter. In one embodiment, the channel may also include a downstream cooling portion having an upstream end communicating with the downstream end of the conversion stage and terminating at the downstream end of the channel.
Cooling in accordance with the present invention maintains the gaseous stream at a temperature which will sustain conversion of the SO2 to SO3 substantially continuously, from the upstream end to the downstream end of the conversion stage and until the conversion of SO2 and SO3 exceeds 95%, typically producing a yield of 97% SO3 or more; a method in accordance with the present invention produces a yield which approaches (i.e., exceeds 99% of) the maximum theoretical conversion percentage, producing a yield of 99%, for example.
Because there is no cooling device between the sulfur burner and the converter, and because there is no cooling device, external of the converter, for cooling the gaseous mixture undergoing conversion, the sulfur burner vessel and the converter vessel can be positioned relatively close together compared to the distance between these vessels in an apparatus employing such cooling devices. Similarly, the length of the conduit between the sulfur burner and the converters is correspondingly small. This reduces substantially the space occupied by the whole of the SO3-producing package, which is desirable.
The present invention may be employed over a wide range of SO2 concentrations, e.g., 4-12%. (As used herein, when SO2 and SO3 contents are expressed as per cents, the per cents are volume per cents.) Equipment and processing expedients which can be employed to produce SO2 concentrations on the high side of the aforementioned range (and higher) are described in the two patent publications identified above. The higher the SO2 concentration, the smaller the volume of the processing equipment and piping needed to handle the gaseous stream containing the SO2. When, as here, the percentage of SO2 converted to SO3 is high (e.g. 97% and higher), the SO3 concentration is essentially the same as the SO2 concentration, and absent dilution of the SO3 with cooling air, the volume of the processing equipment and piping needed to handle the gaseous stream containing the SO3 is relatively small. The smaller the volume of the processing equipment and piping needed to handle the gaseous streams, the smaller the capital expense and the smaller the space occupied by the processing equipment, all of which is desirable.